"it's only 15 credits, but i'm taking the second level of chemistry, biology, anatomy, and psychology.."
- I have no way of evaluating except that my D. would have never signed up for this combo. Her goal was always 2 hard classes and the rest easy. She would consider all (except for chemistry), hard. You may have a different opinion. If 2 out of 4 are easy for YOU, then you are OK. If more than 2 are hard, it may be a problem. On the other hand, I do not believe that D. ever had 15 credits, even in her ligher schedule (when she studied for MCAT and was going to Med. School interviews), the least that she ever had was 16. We do not know the rest of your schedule. Do you work / intern at Research Lab / volunteer / participate in sport, Greek / how you like to spend your weekends. Some of these may be flexible (weekends, Greek, sport) and others require strong commitment (work, lab, even volunteering may have inflexilbe schedule)
Nobody really can tell, you have to evaluate yourself.

How does chemistry + lab, biology + lab, anatomy + lab and psych come out to below average credit hours? 16 is average, right? Excuse my ignorance, at Brown all courses are 1 credit regardless of labs and a standard course load is 4 credits so I never internalized thinking about other credit systems.

^it is not like that at non-Brown schools. The course may be 3-4 credit hrs, lab was usually 1, even though some of them are very time consuming. And 12 hrs is a full time, average does not mean anything. One D's class was wooping 6 credits, and the funny thing is that she did not need these credits at all. She just decided to do few fun projects for her trip to New Zealand (in a summer). She said, why not spend nice time with her neice? Well, the projects were worth 6 credits that she did not need.
So, total may be any number and should reflect the load, but sometime it may be actually either easier or harder, it is all very personal.

How are your grades so far? What kind of load have you taken in the past? These are much more relevant questions than what other people think IMHO. My D did take 3 hard science classes with labs, along with an easier class, several times. It was doable, but pretty time consuming, and she was always glad when it was over. She only attempted it, though, after freshman year and knowing how much she could handle at her school. If you have aced your schedules so far (4.0 or very close to it) and can handle more, it should be fine. If you have had to work really hard with only 1 or 2 science classes with labs, maybe it will be too much.

Ah sorry for the late response. I have taken Chemistry the past semester, a broad one that cover both levels of chemistry, so I'm just taking the 2nd level course right now. Basically I cover the material briefly in the current chemistry course that I am taking right now. Biology and Anatomy are new to me, as well as psychology. My GPA for the last semester (16 credits) was 3.65.

^Again only you can assess. For my D. this schedule would be perfect - 2 hard, 2 easy, she would probably add another easy class (her aim was at least 18 credits in freshman/sophomore years). We do not know which ones are hard and which ones are easy for YOU. And we do not know your overall schedule. Work, sport, Greek, volunteering, Research, weekend preferences??? However, keep in mind, that 3.65 is a min. that you can barely afford. Aim at higher GPA.
Everybody else here will have a different answer. We base our answers on our personal (or parental) experiences/set of skills/preferences/time management ability/talents/requirement for sleep (some may require only 5hrs, others absolutely need 10hrs), they are different and they are different from yours.